bug#40859: Clicking the wrap-prefix string inside a button causes an error

[Clément Pit-Claudel]

Hi all,

The following snippet inserts a button with a long label and a wrap-prefix.  
Clicking the prefix causes an error, whose backtrace is shown below.

(with-current-buffer (get-buffer-create "*button*")
  (erase-buffer)
  (insert-text-button
   (let ((str "long long long long text "))
     (concat str str str str str))
   'follow-link t
   'action (lambda (&rest args) (print args))
   'wrap-prefix "this is the wrap prefix")
  (pop-to-buffer (current-buffer)))

Debugger entered--Lisp error: (void-function nil)
  nil(("this is the wrap prefix" . 9))
  button-activate(("this is the wrap prefix" . 9) t)
  push-button((mouse-2 (#<window 13 on *button*> 81 (76 . 29) 379842354 ("this 
is the wrap prefix" . 9) 81 (9 . 1) nil (4 . 12) (8 . 17))))
  funcall-interactively(push-button (mouse-2 (#<window 13 on *button*> 81 (76 . 
29) 379842354 ("this is the wrap prefix" . 9) 81 (9 . 1) nil (4 . 12) (8 . 
17))))
  call-interactively(push-button nil nil)
  command-execute(push-button)

Cheers,
Clément.

In GNU Emacs 28.0.50 (build 5, x86_64-pc-linux-gnu, GTK+ Version 3.22.30, cairo 
version 1.15.10)
 of 2020-03-15 built on clem-w50-mint
Repository revision: 9dccaf8a5cdb10dae597345ec3741475477a7d97
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description: Linux Mint 19.3

Recent messages:
Mark set
#<buffer *button*>
button-activate: Symbol’s function definition is void: nil
Debug on Error enabled globally
Entering debugger...
Mark set [2 times]
Saved text until "nil nil)
  command-execute(push-button)
"
Mark set

Configured features:
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Major mode: Lisp Interaction

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Load-path shadows:
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Features:
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Memory information:
((conses 16 57304 5023)
 (symbols 48 7631 1)
 (strings 32 18904 1618)
 (string-bytes 1 584373)
 (vectors 16 10294)
 (vector-slots 8 143875 8980)
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